Aerosols: The Last Frontier?

The recently released IPCC 2007 Fourth Assessment Report Summary for Policymakers reminds us that aerosols remain the least understood component of the climate system. Aerosols are solid or liquid particles suspended in the atmosphere, consisting of (in rough order of abundance): sea salt, mineral dust, inorganic salts such as ammonium sulfate (which has natural as well as anthropogenic sources from e.g. coal burning), and carbonaceous aerosol such as soot, plant emissions, and incompletely combusted fossil fuel. As should be apparent from this list, there are many natural sources of aerosol, but changes have been observed in particular, in the atmospheric loading of carbonaceous aerosol and sulphates, which originate in part from fossil fuel burning. While a relatively minor part of the overall aerosol mass, changes in the anthropogenic portion of aerosols since 1750 have resulted in a globally averaged net radiative forcing of roughly -1.2 W/m2, in comparison to the overall average CO2 forcing of +1.66 W/m2.

Figure SPM-2, shown here, compares the radiative forcing for greenhouse gases and other climate forcing agents, along with an assessment of the level of scientific understanding (“LOSU”) for each component. In this figure, it is clear that while aerosols contribute the largest negative (cooling) radiative forcing, the level of scientific understanding of their climate influence is “low” to “medium-low”. The aerosol effects are split into two categories: (1) direct effects, meaning the scattering or absorption of radiation by aerosols influencing the net amount of energy reaching the Earth’s surface, and (2) indirect effects, such as the cloud albedo effect, referring to how the presence of aerosol increases cloud reflectivity by providing a larger number of nuclei for cloud droplets, reducing the amount of energy reaching the surface. This is a step up from the last report, where the LOSU for aerosols was very low to low, and no most likely value was assigned at all for the ‘indirect’ part.

This figure also visually hints at why improving our understanding of aerosol’s role in climate is so important: while overall net radiative forcing is positive (warming), aerosols provide the dominant negative (cooling) forcings. Hence, the aerosol currently in our atmosphere is acting to mask some of the greenhouse gas-induced warming. This means that as we get our act together to reduce fossil fuel use to improve air quality and address global warming, we need to be mindful of how changes in emissions will impact aerosol concentrations and composition.

In addition, our deficient understanding of aerosol forcing also hinders our ability to use the modern temperature record to constrain the “climate sensitivity” – the operative parameter in determining exactly how much warming will result from a given increase in CO2 concentration. The determination of climate sensitivity has been discussed in this forum previously here. The sensitivity parameter can be derived by examining historical records of the correlation of CO2 concentration and temperature taking into account other contemporary changes. Aerosols contribute significantly to the uncertainty in climate sensitivity because we cannot model their historical impact on the temperature record with sufficient accuracy, though additional constraints on climate sensitivity such as the last ice age do exist. A better understanding of aerosols then may well facilitate more accurate predictions of future climate responses to changing CO2.

The relative lifetimes of CO2 and aerosol in the atmosphere result in the expectation that reducing fossil fuel use will accelerate warming. A CO2 molecule has a lifetime of about 100 years in the atmosphere, while an aerosol particle has an average life expectancy of only about 10 days. Therefore, if we instantaneously ceased using combustion engines, the (cooling) fossil fuel-related aerosols would be cleaned out of the atmosphere within weeks, while the (warming) CO2 would remain much longer, leaving a net positive forcing from the reduction in emissions for a century or more.

So, what do we need to learn about aerosol to narrow those error bars in Figure SPM-2? To accurately model aerosols’ climate impact, we need to know about the whole lifespan of the aerosols: their diverse sources, aging processes (and how those affect radiative properties), how they mix together and the mechanisms and timescales for its removal from the atmosphere. As the IPCC 2007 4AR will make clear, we’ve come a long way in our understanding of atmospheric aerosol, but there is still plenty of room for improvement.

184 Responses to “Aerosols: The Last Frontier?”

#146 & 148, only for the sake of argument, suppose the climate is less sensitive to GHGs and the sun is causing the major portion of the warming, we can do nothing about the sun, but we can do something about our GHGs. And whether it’s the sun or us, GW is a dangerous thing. So we have to do what we can to reduce it. Since in this scenario the climate is not very sensitive to GHGs, that means we will have to reduce even more to achieve some reduction in GW.

But the closer truth be known, the climate is probably more sensitive to our GHGs, taking the aerosol-effect into account AND the possible positive feedbacks in the future, so in this scenario, too, we must drastically reduce our GHGs.

Which brings up a point that I think is too often ignored by the advocates of solar power, which is that most of the incoming solar energy is already being used by plants. Cover the earth, or even a sizeable fraction of it, with solar panels, and you’ve just taken away the energy source that drives most of the biosphere.

The same is true, in less degree, of commercial solar plants. Intercept the sunlight, and the plants underneath will die. At what point does that start causing its own environmental problems?

Re #149. Renewed thanks – and 10-15 years for a net negative forcing from ending fossil fuel use looks a lot better to me than 100 or more! Interestingly, even amusingly insofar as anything this serious has a funny side, the reduction in GHG emissions the UK government likes to boast about comes almost entirely from the switch from burning coal to burning gas in power stations (a switch which was motivated by cost-saving, not GHG emission reduction). Indeed if it wasn’t for this switch, UK GHG emissions would have increased considerably over the period from 1990. If your calculations are roughly correct, we’ve not yet reached net negative forcing from the change from coal to gas.

[[Which brings up a point that I think is too often ignored by the advocates of solar power, which is that most of the incoming solar energy is already being used by plants. Cover the earth, or even a sizeable fraction of it, with solar panels, and you’ve just taken away the energy source that drives most of the biosphere.

The same is true, in less degree, of commercial solar plants. Intercept the sunlight, and the plants underneath will die. At what point does that start causing its own environmental problems? ]]

The “sizeable fraction” necessary is less than 1% of Earth’s land surface. The plants will survive.

Re #148, Ray’s comment: I thought we were at the high point of the solar cycle, however significant that may be. Future changes in solar output are more likely to have a cooling effect. However, at most that would be about half a watt per square meter, equivalent to a few decades of warming at the current rate.

So if we are lucky we might get some extra time to reduce anthropogenic warming, but I would not count on it.

#136 Gavin comment : remember that a large number of us have authored papers on solar changes and possible responses

Not only I remember… but I read you! I’ve just finished Rasmus’ book on that topic.

#148 Raypierre comment : Since there isn’t any evidence that would justify increasing the estimate of the solar contribution in the past, this line of thinking is more or less moot anyway

It would be interesting to know if there’s any solar influence on XXth century, in your opinion (I mean of course a response to forcing, not the natural influence day/night, seasons, etc.). After all, with 0,1 W/m2 TOA forcing 1750-2000, it should be nearly impossible to discern a distinct signature on surface temperatures. An equilibrium climate sensitivity of 0,75 K.W/m2 implies in this case a 0,075 K response, inferior to the margin error / uncertainty of instrumental measurement on the Hadley CRU database. What I miss is the nature and relative weight of forcing agents for 1750-1950 warming (because if there’s no solar trend since 1950, the solar forcing 1750-1950 is also 0,1 W/m2). Anyway, that’s off topic here, I’ll go on with my readings and try to understand.

What Raypierre alludes too is the long-term influence of solar. While there is less doubt about the variation in energy (TOA) over the last two cycles (+/- 0.5 W/m2), there still is a lot of discussion about the influence of solar in the longer past (and even over change of the last cycle minima). What is clear, is that we now are at an exceptional high level of solar activity. There is a lot of discussion going on in the solar community what the next cycle will bring (some think still high, others predict a much smaller maximum).
In general the change in solar forcing from the LIA to present is thought to be in the order of a few times the variation over one cycle. How much that change influences temperature is the most controversial point and is highly dependent of the reconstruction used.
Reconstructions with a low variation (like MBH98/99) indicate a small influence of solar, as volcanic eruptions cause a cooling of (at maximum) 0.1 K over the past 600 years, leaving 0.1 K for natural (mainly solar) influences. Reconstructions with high variation (like Moberg, bore holes) have the same influence of volcanic, but up to 0.7 K for natural.

This result of this is that in the first case, solar changes had a small influence on the temperature increase of the 20th century (10-20%) and thus in the future, in the second case, it is up to 50% (as Scafetti tried to explain in a previous discussion). The rest of the warming is mostly from GHGs, as far as internal natural cycles are not interfering. I am pretty convinced of the high influence of solar, but that is only possible if responses to solar forcing differs from responses to GHGs (and are much higher), which is possible, as solar (and volcanic) have their highest influence in the stratosphere, while GHGs (and anthro aerosols) have their highest influence in the (lower) troposphere. That is my difference in opinion with Raypierre and Gavin (and most climate modellers), who see only small differences in sensitivity for the four main forcings…

If solar has more influence than currently implied in models, that means that we may have more time to change to non-fossil fuels, as for the same amount of CO2, the resulting temperature increase is lower…

Re #157, 158: Perhaps my figures are a little out of date; they are from Lean and Rind 1998. The recent IPCC SPF states “Changes in solar irradiance since 1750 are estimated to cause a radiative forcing of +0.12 [+0.06 to +0.30] W m-2, which is less than half the estimate given in the TAR.”

[[If solar has more influence than currently implied in models, that means that we may have more time to change to non-fossil fuels, as for the same amount of CO2, the resulting temperature increase is lower… ]]

Sigh…

For the hundredth time: the extent of the Solar forcing has no effect on the extent of the CO2 forcing. They are independent.

Solar can’t be driving the present global warming because A) the Sun’s luminosity hasn’t varied noticeably in 50 years as measured by satellites; B) increased sunlight would heat the stratosphere, and the present stratosphere is cooling; and C) increased sunlight would heat the equator more than the poles, whereas we are now seeing much greater warming at high latitudes.

#161 Barton, I don’t agree with your three assertions, but please correct me if I’m wrong.

A) Solar total and spectral irradiance is measured by satellite since 1979, not 50 years. And there’s still a debate for the interpretation of cycle 21-23 data.

B) For this 1979-2006 period, the ozone depletion is supposed to greatly influence the stratos. temperature trends, as measured by satellites in lower layers (14-22 km). Anyway, the 1979-2006 TSI trend is weak if any, so you’re not supposed to get a clear signature here, nor in upper strato. layers 22-50 km.

C) I sea no reason for a major solar influence on Equator surface temperatures rather than mid and high latitudes ones. Models usually show that climate change of the past are more pronounced poleward, whatever the forcing. Heat in intertropical zone is redistributed either by convection in upper layer or by oceanic / atmospheric circulation in higher latitudes. (Maunder Minimum, whose everybody agree it was partly solar induced, was mainly marked on NH lands, not on Tropics, for example).

For the first point, I’m not sure sun doesn’t influence CO2 forcing, or at least climate feedbacks to CO2 forcing. For example, CO2 effects won’t be exactly the same in a 1360 or 1365 W/m2 TSI situation, so far the (relative) forcing of GHGs depends on water vapour concentrations, emissivity temp. of each layer, ocean and vegetal carbon sink in response to luminosity, etc.

#163 AFIAR, there’s a solar signal in tropical atmosphere of minimum-to-maximum amplitude of a cycle, mainly observed in high to mid troposphere. But we’re speaking here of solar forcing, not relative insolation, that is long term effects of a radiative imbalance on climate. And I never read that climate models expect a pronounced tropical response to multidecadal TSI increase / decrease. Any reference for that? If you look at IPCC 2001 “6.14 The Geographical Distribution of the Radiative Forcings”, you see that GHGs direct forcing is more pronounced in low than in high latitudes. Do you expect a maximal response to GHGs forcing at 30Â°N / 30Â°S?

Barton, in addition to what Charles said, the discussion is not about forcings, it is about sensitivity, or the effect of forcings + feedbacks on temperature. While forcings are more or less known for GHGs and recently for solar strength (still with a lot of discussion), feedbacks may be different, especially for cloud formation, which scientific understanding is very low. And responsible for much of the wide range in projection for 2xCO2. 2xCO2 without feedbacks only increases global temperature with 0.85 K, the rest is thought to be from water vapor feedback and other feedbacks (ice albedo, clouds). Thus if the sensitivity (and the forcing for the pre-satellite period) for solar is higher than implemented in current GCM’s, that is at the cost of the sensitivity of GHGs (not the forcing) and the sensitivity and/or forcing of aerosols, as these work in tandem.

Besides the stratospheric effects on climate (variations in jet stream position and rain over the US and South Europe), global cloudiness varies with +/- 2% over the 11-year cycle. Solar has its highest energy effect in the tropics (but the heat is redistributed towards the poles), while the effect of GHGs is more evenly spread over the globe.

Ocean heat content increase was highest in the subtropics, where a reduction in cloud cover was measured (+ 2 W/m2 more insolation in 15 years time over the whole tropics). If the latter is internal natural or solar induced, that is an unsolved question…

Funny about the figure given for co2 logevity. You say a century while the figure commenly given is several centuries. One century would mean that co2 molecules emitted before 1906 would have left the atmosphere by now. What is the correct span?

It is well known that the warming of sea surface temperatures amplifies energy transfer to the atmosphere which in turn, increases the frequency and power of hurricanes. Would it then be right to assume that this can also cause a corresponding increase in the transfer of sea salt to the upper atmophere? I have read accounts of whole schools of fish being drawn aloft and dumped many miles away.

Re #167: The 100 year lifetime for CO2 residence time in the atmosphere is rather arbitrary. In 20 years almost half of it is gone, in 100 years 25% of it still remains, and in 100,000 years 7% is still there. See the graph on this page.

In response to #169, you are dead on about one thing: hurricanes do have the ability to convectively transport aerosols up away from the boundary layer to the free troposphere. (sea salt in cirrus clouds in mid-America). Also, evidence has shown that aerosols from tropical fires reach the upper troposphere via deep tropical convection, where they are free to get swept in the strong upper level winds, and carried great distances. (tropical aerosols and convection) However, I am not sure that the assumption that warming-induced increases in hurricane intensity will make a big impact on the amount of aerosols in the troposphere, mostly because over the ocean (where hurricanes are) sea-salt aerosols are abundant, and that in order for them to get swept into the hurricane in the first place, they must have been made airborne by some other process. It is plausible to associate that “other process” to increased wave breaking in hurricanes, but I have a hard time believing that this would transport significantly more sea salt to the atmosphere.

But your assumptions are definitely valid, and I think they play into a more dire problem: tropical forest fires. Anthropogenic forest fires have increased dramatically in recent decades, especially in the tropics. The input of aerosols from these fires is enormous, and can result in a multitude of climate responses. Understanding the transport of aerosols to the upper troposphere via convection is important, and I think more needs to be understood of the indirect effects of aerosols on clouds. One recent study has shown that subtropical absorbing aerosols (soot) actually decreased cloud cover (specifically trade cumulus) by heating the top of the boundary layer. (reference Ackerman, et al., 2000) This effect on aerosols would not mute the ongoing effects of CO2, but rather enhance it. Any thoughts on this?

That is nuts what you said. An engineer is creationist eh? You know as a guy with a Master’s in Electrical Engineering and a very strong minor in modern physics I take exception to that. You simply don’t know your rear from your elbow.
I used to work the Los Alamos National High Magnetic Field Laboratory. (Only dumb people work there, you know guys from MIT,UFL,FSU, Georgia Tech, Caltech etc.etc.) I met a Nobel Prize winning physicist there and he was a lot more humble than you are and made some important discoveries in his time there. I also met a lot of physicsts that could not find their way to the bathroom. Should I generalize about physicists as well? Would not that make them look very stupid?

re: 174. Your ad homs aside, all of that you stated clearly indicates that you are not an expert in climate models, science and similar research. As opposed to the literally thousands of climate scientists around the world and essentially every major scientific society around the world who agrees regarding the influence of man-made GHG emissions on climate. And who have published their research in peer-reviewed journals.

Your comments aside, that was not the point. Never claimed to be a climate scientest and I don’t comment about them either. You clearly can’t read well because I don’t comment often and it is on subjects I know something about. I do know power systems, digital design (mostly dealing with ASIC design), communcations, signal processing and such as that. Do you? If not then keep silent about subject matters that you also know nothing about. In any case that was directed at Barton. If you have guilty heart fine. Also Dan are you an “expert”, or are you a self educated google user?
Further, it clearly was not for fun. The generalization made by Barton is that engineers were not well educated in science and that is simply false.

[[That is nuts what you said. An engineer is creationist eh? You know as a guy with a Master’s in Electrical Engineering and a very strong minor in modern physics I take exception to that. You simply don’t know your rear from your elbow.]]

I don’t think you understood what I said; perhaps I phrased it poorly. It has been my experience that engineers are over-represented among creationists and Velikovsky followers, and I attribute that to their believing that an ability to manipulate equations makes them scientists. But you can’t get “most engineers are creationists” from “some creationists are engineers.” It would be a non sequitur.

“It didn’t surprise me to find out that a lot of creationists and believers in Velikovsky’s astronomy are engineers. They think if you can manipulate equations, that makes you a scientist”

Will make any engineer angry. In any case engineering is built on physics and mathematics and to say that engineers are not scientists is silly. In any case as a guy with a couple of classes short of a physics degree to get my Eletrical Engineering B.S and M.S it is even more infuriating. I could say (truthfully too!) that most of the visiting physcists at the Los Alamos National High Magnetic Field Laboratory were severly lacking in common sense and were trapped by in-the-box thinking. In essence they could not make anything work. I believe that physicists have great ideas but can’t apply them or make them work in the real world. Whereas engineers were more interested in applying science to the real world in order to obtain real results. At least that was my experience at LANL NHMFL.

That is simply wrong.
I worked with a lot of physicists and they had their eye on developing products/process with their ideas. Materials science and biophysics come to mind as immediate examples of this. To say that what makes a scientist is to not have a wish to commercially develop their ideas is naive in the extreme. (I guess all of those royalties from patents that professors and universities recieve is from charity eh?) You sound almost religous about a scientists intentions!

In any case as a grad engineering student I published papers that were submitted to peer review all of the time. My professors were so much more so. They were constantly going to conferences and reviewing other’s works, inventing new process and new ideas. (The “peers” were from Comp Sci, Mathematics, Physics, etc.etc.) I say again Engineers are taught physics just like physicists are and just as well. Lot of guys in the same classes you know, same teachers. And how different can EM field mechanics, fluid dynamics, and thermodynamics be between engineers and physicists?

Water vapor feedback may hold a more significant unknown. The level of feedback due to visible absorption of solar radiation is not well understood and not well accepted. Reports have suggested that the feedback is stronger than calculated, and Arking has suggested some time ago that shortwave quadratic dependent water absorption is the explaination. Recent as yet unpublished measurements support his theory.